88 Field Instrumentation GEN Rev) Plow = Patm P = Phigh - Plow = SH + S⋅X That is, the pressure on the high-pressure side is always higher than the actual pressure exerted by the liquid column in the tank (by a value of S⋅X). This constant pressure would cause an output signal that is higher than 4 mA when the tank is empty and above 20 mA when it is full. The transmitter has to be negatively biased by a value of -S⋅X so that the output of the transmitter is proportional to the tank level (SH) only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter. A zero suppression kit can be installed in the transmitter for this purpose.
Zero Elevation When a wet leg installation is used (see Figure 7 below, the low-pressure side of the level transmitter will always experience a higher pressure than the high-pressure side. This is due to the fact that the height of the wet leg (Xis always equal to or greater than the maximum height of the liquid column (H) inside the tank. When the liquid level is at H meters, we have
Phigh = Pgas + SH Plow = Pgas + S⋅X P = Phigh - Plow = SH - S⋅X
= - S (X - H) The differential pressure P sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than high pressure side. P increases from P = -S⋅X to PS (X-H) as the tank level rises from 0% to 100%. If the transmitter were not calibrated for this constant negative error (-S⋅X), the transmitter output would read low at all times. To properly
calibrate the transmitter, a positive bias (+S⋅X) is needed to elevate the transmitter output. This positive biasing technique is called zero elevation.
89 Field Instrumentation GEN Rev)
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